Two Charts: Not One

Quality Engineering vol. 19 issue 2 - April 2007

Abstract: [This abstract is based on the authors' abstract.]Because data coming from a typical operating process do not vary about a fixed mean, the operation of traditional control charts may be inconsistent. It is shown that a nonstationary model provides a continuously updated local mean level that can be used to produce both a bounded adjustment chart for determining when to adjust the process to achieve maximum economy and a Shewhart monitoring chart identifying assignable causes of trouble applied to the deviations from the local mean. Estimation of the mean and standard deviation are not required.

Keywords: Exponentially weighted moving average control charts (EWMA) - ARIMA time series models - Monitoring - Noise - Design of control charts - Shewhart control chart - Stationary process

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